This invention generally relates to casting processes and materials. More particularly, the invention relates to core and mold materials and processes for casting reactive metal alloys, including reactive metal alloys containing one or more reinforcing intermetallic phases, such as a niobium-silicide alloy.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. While nickel, cobalt and iron-base superalloys have found wide use for components within the hot sections of gas turbine engines, alternative materials have been proposed to achieve improved high-temperature properties. For example, silicon-based non-oxide ceramics, such as silicon carbide (SiC) as a matrix and/or as a reinforcing material, have been proposed as candidates for high temperature applications, such as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines. Another example is refractory metal intermetallic composite (RMIC) materials, including alloys based on niobium, titanium, hafnium and zirconium, a notable example of which is niobium-silicon (NbSi) alloys. These alloys may further contain other alloying constituents, including titanium, hafnium, aluminum, chromium, etc. Depending on the particular composition, RMIC materials typically melt within a range of about 1500° C. to about 2100° C., and as such can have much greater temperature capabilities than existing nickel, cobalt, and iron-based superalloys.
NbSi alloys have become of particular interest for replacing superalloys in the hot sections of turbine engines because they are capable of possessing a promising combination of low-temperature mechanical properties, such as room temperature toughness, as well as high-temperature strength and creep resistance. The NbSi alloys usually have a microstructure containing a metallic Nb-base phase and one or more intermetallic phases including intermetallic niobium-silicide that form in-situ during solidification of the alloy. The niobium-base phase is relatively ductile, while the intermetallic niobium-silicide phase is brittle and stronger. As such, NbSi alloys may be referred to as a composite of a ductile metallic matrix phase and a brittle intermetallic strengthening phase.
As with superalloys, RMIC materials have been formed into gas turbine engine components by various casting techniques, a notable example being investment casting (lost wax) processes. Investment casting typically entails dipping a wax or plastic model or pattern of the desired component into a slurry comprising a binder and a refractory particulate material to form a slurry layer on the pattern. A common material for the binder is a silica-based material, for example, colloidal silica. A stucco coating of a refractory particulate material is typically applied to the surface of the slurry layer, after which the slurry/stucco coating is dried. The preceding steps may be repeated any number of times to form a shell mold of suitable thickness around the wax pattern. The wax pattern can then be eliminated from the shell mold, such as by heating, after which the mold is fired to sinter the refractory particulate material and achieve a suitable strength.
To produce hollow components, such as turbine blades and vanes having intricate air-cooling channels, one or more cores must be positioned within the shell mold to define the cooling channels and any other required internal features. Cores are typically made using a plasticized ceramic mixture that is injection molded or transfer molded in a die or mold, and then hardened by firing or baking. Typical ceramic compositions contain silica and/or alumina. One or more fired cores are then positioned within a pattern die cavity into which a wax, plastic or other suitably low-melting material is introduced to form the pattern. The pattern with its internal core(s) can then be used to form a shell mold as described above. Once the shell mold is completed and the pattern selectively removed to leave the shell mold and core(s), the shell mold can be filled with a molten metal, which is then allowed to solidify to form the desired component. The mold and core are then removed to leave the cast component with one or more internal passages where the core(s) formerly resided. Removal of silica-based and alumina-based cores is performed by a leaching process with a caustic solution (typically aqueous solutions of NaOH or KOH) in an autoclave at high pressures (e.g., about 100 to 500 psi; about 0.7 to 3.5 MPa) and temperatures (e.g., about 200° C.), with typically treatments requiring about ten to twenty hours, depending on the size of the core.
From the above, it can be appreciated that shell molds and cores used in investment casting processes must exhibit sufficient strength and integrity to ensure that the component will have the required dimensions, including wall thicknesses resulting from the location of each core relative to the shell mold. Additional challenges are encountered when attempting to form hollow castings of reactive materials, including NbSi alloys, as a result of their high melting temperatures and reactivity. Several approaches have been attempted to produce NbSi alloys, including casting, directional solidification, powder metallurgy, vapor deposition, and ingot metallurgy with thermomechanical processing. However, the high reactivity and complex chemistries of NbSi alloys have presented significant barriers to investment casting with conventional ceramic molds, as conventional silica and alumina cores have been found to react with NbSi alloys at the high casting temperatures.
Facecoats have been proposed to form a protective barrier between the molten NbSi alloy and the surfaces of the mold. For example, commonly-assigned U.S. Pat. No. 6,676,381 to Subramanian et al. describes a facecoat based on at least one rare-earth metal oxide and other inorganic components, including silicides, silicates, sulfides, and other oxides. Facecoats can also be used to protect surfaces of cores within a shell mold. However, improved materials for shell molds and cores are necessary in order to render investment casting processes viable for casting hollow near-net shaped articles of NbSi alloys.